Being apex predators in the North Pacific, elephant seals accumulate high concentrations of mercury in their bodies, and when they shed their fur during their annual molt, they offload that toxic metal too. The findings, published in Proceedings of the National Academy of Sciences this week, suggest that the marine mammals might be contributing significant amounts of the environmental contaminant to coastal ecosystems.
The most toxic form of mercury, methylmercury (MeHg), is a potent neurotoxin that’s readily absorbed into the bodies of marine organisms. And through a process called biomagnification, it becomes more and more concentrated as it passes up the food chain. That means mercury concentrations in aquatic carnivores are 1 million to 10 million times higher than the levels found in seawater.
Excreta and fur shed by northern elephant seals (Mirounga angustirostris) are thought to constitute a source of MeHg contamination at the base of marine food chains. Previous work revealed high concentrations of mercury in sea lion feces, but because elephant seals fast during most of their time on the beach, they’re not excreting much at all. And there’s also a seasonal element to MeHg levels – suggesting that the main source might be seal fur. Elephant seals undergo an annual "catastrophic molt," shedding the entire outer layer of skin and hair. This so-called molted pelage comes off in sheets onto the beach.
To investigate, a trio from the University of California at Santa Cruz (UCSC) led by Jennifer Cossaboon collected seawater near the northern elephant seal rookery at Año Nuevo State Reserve in California. They compared the MeHg concentrations to that of other coastal sites where there aren’t large marine mammal populations. "Many studies have looked at biomagnification up the food chain, and we took that a step further to see what happens next," Cossaboon said in a statement. "Mercury is an element, so it never breaks down and goes away – it just changes forms."
The team discovered that MeHg concentrations were significantly higher at Año Nuevo than at the other sites during both breeding and molting seasons, but especially during the latter: MeHg levels were twice as high during breeding season and 17 times higher during molting season. Furthermore, the team found high mercury concentrations in the molted pelage samples they collected at Año Nuevo.
Coal burning and other such industrial emissions have significantly increased the amount of mercury in the marine environment. "This internal recycling back into the coastal environment just adds to the problem," study coauthor Russell Flegal of UCSC adds. Methyl mercury levels in nearshore waters during molting season were greater than those found in even the highly urbanized San Francisco Bay estuary.